Bump MSRV to 1.61

```
error: package `memchr v2.6.3` cannot be built because it requires rustc 1.61 or newer, while the currently active rustc version is 1.60.0
```

Author: taiki-e

.github/workflows/ci.yml

@@ -50,7 +50,7 @@ jobs:
       matrix:
         # When updating this, the reminder to update the minimum supported
         # Rust version in Cargo.toml.
-        rust: ['1.48']
+        rust: ['1.61']
     steps:
       - uses: actions/checkout@v3
       - name: Install Rust

Cargo.toml

@@ -6,7 +6,7 @@ name = "async-executor"
 version = "1.5.2"
 authors = ["Stjepan Glavina <[email protected]>"]
 edition = "2018"
-rust-version = "1.48"
+rust-version = "1.61"
 description = "Async executor"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/smol-rs/async-executor"